In order to meet the most demanding requirements of energy efficiency and cooling, domestic and commercial cooling systems have the option of using variable capacity compressors, which allow the adjustment of the cooling capacity by varying the speed of pumping the coolant gas (that is, the mass flow), in accordance with the system's need and its demand for cooling.
Said variable capacity compressor performs the excursion of a minimum value of mass flow to a maximum value by varying the rotation of its motor. Rotation variation is obtained by means of an electronic control called frequency inverter, which adjusts the voltage and the frequency applied to the motor.
Said frequency inverter is composed of various electronic circuits having distinct functions, such as, for example, a power circuit that has an input stage for electromagnetic interference filtering and a stage called “bridge rectifier” for converting the alternating current of the power grid to continuous voltage, a control circuit (microcontroller or DSP—Digital Signal Processor), an auxiliary power supply for generating internal voltages for the other circuits or components of the inverter, a circuit formed by power semiconductors to drive the electric motor employed in the compressor, among others.
FIG. 1 shows, in simplified form, the main components of a frequency inverter according to the state of the art, applied to variable capacity compressors 60. Connected to the frequency inverter are an alternating current power grid 50 and a thermostat of the cooler at its inputs, and the compressor at its output, whose running it controls. The main components of the power circuit 11 of the inverter are the electromagnetic interference filter, a diode bridge rectifier, the CB bus capacitor and the three-phase inverter bridge. The voltage on the CB capacitor is the result of rectification of the input alternating current, forming the CC bus of the inverter, to which there is connected an auxiliary voltage supply. This supply is responsible for providing the feed voltage to the other components of the inverter, such as command, communication and control circuits of the frequency inverter, converting the continuous high voltage, generated on the CC bus of the power circuit, into continuous low amplitude voltage, suitable for feeding these components. Once these subcircuits are powered up, they work as usual, and the command signal is received from the thermostat, which is interpreted by the control circuit which drives the inverter bridge and monitors the electrical magnitudes of the compressor in order to control it. It can be seen in FIG. 1 that the auxiliary power supply is permanently connected to the CC bus, consuming energy, regardless of the status of the compressor (on or off).
The auxiliary power supply may employ high frequency energy conversion methods, commonly called “SMPS”—Switched Mode Power Supply, or low frequency methods, such as linear supplies and capacitive supplies. Regardless of the topology of the auxiliary power supply, in the state of the art it is continuously connected to the CC bus, whether or not the compressor is running. Unless the inverter is disconnected from the alternating current of the power grid, this supply will be continually consuming energy, of a few Wh (Watt-hour) or hundreds of mWh (milli-Watt-hour). The consumption of energy during compressor downtime is called “Stand-By Consumption” and its function can basically be resumed in maintaining the control circuit of frequency inverter prepared to drive the compressor again in a new cooling cycle of the cooling system.
The stand-by consumption, though small compared to the consumption of energy while the compressor is operating, is considered undesirable since it represents a waste of energy for a time interval in which the cooling system compressor is not performing its main function of removing heat from the cooling system by movement and compression of coolant gas. The stand-by consumption of the frequency inverter is, therefore, a source of energy losses in a cooling system, as it is an absolutely dispensable waste.
With the objective of increasing the efficiency of cooling systems, in the state of the art, the frequency inverter is disconnected from the power grid whenever the compressor is inactive, disconnecting the auxiliary power supply and eliminating the stand-by consumption. This method employs switches, such as relays or electro-mechanical thermostats, or semiconductors. In both cases, the switches are dimensioned to be able to withstand the input electric current of the frequency inverter, of high amplitude when the compressor is in operation. In the case of using relays, there is also the drawback of having consumption by this relay during the interval in which the compressor is active, whereby minimizing the gain obtained by disconnecting the inverter from the power grid and consequent elimination of the stand-by consumption. Further, disconnection by relay, when performed by another electronic control present in the cooling system (control referred to as “electronic thermostat”), requires that this second control have an oversized the electronic circuit, such as, for example, the presence of a digital output to drive the relay, the relay itself, and a power supply capable of driving this relay during the interval in which the compressor is in operation. In contrast, the use of semiconductors, such as TRIACs, to interrupt the feed of the frequency inverter, also has the drawbacks of conduction losses and the need to oversize this semiconductor to withstand the initial charge current from the CB bus capacitor of the inverter (in-rush current).
FIGS. 2a, 2b and 2c represent arrangements of the inverter according to the state of the art. As will be noted in the descriptions ahead, in none of the cases is the command signal, or the physical interface means (cables) between the inverter and an external thermostat, used to disconnect the auxiliary supply of the inverter.
FIG. 2a represents a cooling system arrangement 1, wherein an electronic thermostat 2 has a control circuit 4 responsible for defining the operating status of the compressor. The electronic thermostat sends command signals to the frequency inverter 3 through the cables 8. The inverter has a circuit 5 responsible for receiving the signal from the thermostat and adjusting it to interpret the control circuit 6, which controls the operation of the compressor, which can be called communication unit. In this arrangement, the command signal of the thermostat may assume different formats, according to the communication protocol of each manufacturer of cooling systems. For example, it is possible to send a frequency signal proportional to the rotation which is desirable for the compressor, a certain frequency value or the absence of a signal (zero) to keep the compressor disconnected, among others. It is noted that this cooling system has a cable 8 for communication between the thermostat and the communication circuit 5. This connection is used by the thermostat to send operation control signals of the compressor, for example, references for frequency and amplitude from the feed signal of the compressor. Although the figure shows two links between the thermostat and the communication circuit, one is the reference (zero) and the other is the signal itself, to the extent that there is only one cable.
FIG. 2b shows another arrangement of the compressor control system according to the state of the art, in which the control circuit 4 of the electronic thermostat 2 drives a switch 9 to send to the input circuit 5 of the frequency inverter a voltage signal referenced to the power grid. In other words, the control circuit 6 of the frequency inverter 3 receives pulses with the same frequency of the power grid. The thermostat sends to the communication input circuit 5 both a signal to command the connection/disconnection of the compressor, and command signals obtained by modulating the switch 9, though the latter are less usual. The switch 9 may be both an electro-mechanical relay contact and a semiconductor referenced to the power grid, and usually it is maintained open when it is desirable for the compressor to be disconnected.
FIG. 2c shows a simpler arrangement of the compressor control system according to the state of the art. Here, the thermostat 10 is not electronic, but of the electro-mechanical kind. The thermostat has a contact that is closed when the temperature of the cooling system rises above the reference value. In this arrangement, the contact of the electro-mechanical thermostat is open whenever it is desirable to keep the compressor disconnected. Both in this arrangement, as in that of FIG. 2b, the rotation of the motor inside the compressor is adjusted by the frequency inverter and not by the thermostat. The only command from the thermostat is to connect or disconnect the compressor. In FIG. 2a and potentially in FIG. 2b, the thermostat sends to the inverter both a signal to connect or disconnect the compressor, and also command signals to control the frequency and/or a feed voltage of the compressor.
In all the arrangements described in the state of the art, the frequency inverter is permanently connected to the alternating current power grid through the cable 7. Therefore, even if the compressor is disconnected, the CB capacitor of the CC bus, shown in FIG. 1, is charged with the rectified voltage from the power grid and the auxiliary power supply of the inverter will be consuming a quantity of energy to keep the control, communication and command circuits of the inverter running, prepared to reconnect the compressor in a subsequent command to connect the thermostat.
FIG. 3 is a graph that shows the input power of the frequency inverter using control systems also from the state of the art. In this graph, two power levels can be seen, one about 40 W which occurs when the compressor is connected, and another about 0.7 W during stand-by, that is, in the intervals in which the compressor is disconnected. In order to reduce energy consumption and increase the efficiency of the cooling system, it is desirable to reduce drastically this value of 0.7 W. In the systems of the state of the art with a consumption behavior such as that illustrated in FIG. 3, the average available power is 24 Wh (40 W at peak, with working cycles of 60%—cycle of 60 minutes, in which the compressor remains connected for 36 minutes). If the stand-by consumption were reduced to 0.1 W, there would be a gain of 0.24 Wh in average consumption, representing an improvement of 1% in the efficiency of the system.
FIG. 4 illustrates a circuit of the state of the art which attempts to eliminate the stand-by consumption of the inverter, using an arrangement in which the frequency inverter 3 is not permanently connected to the alternating current power grid. Here, the phase or the neutral of the power grid is disconnected from the inverter by way of the switch 9 present in the thermostat 2. This switch may be a contact of an electro-mechanical relay or a semiconductor of the TRIAC kind, and it is commanded to open the feed of the inverter whenever it is necessary to disconnect the compressor. When the compressor motor has to be connected, the inverter is again connected to the power grid by closing the circuit by way of the switch. Then, the command for defining the rotation of the motor inside the compressor is sent from the thermostat 2 to the inverter through the cable 8, which is a separate connection from the one that connects and disconnects the inverter. There are also other arrangements similar to the one illustrated in FIG. 2c, in which the thermostat sends commands only to connect or disconnect the inverter, while it controls the rotation of the motor. However, in the latter cases, there must be a non-volatile memory in the inverter, so that it memorizes the prior operating status of the compressor in order to define a rotation status soon after powering up the inverter.
The solutions of the state of the art to eliminate the stand-by consumption provide an unsatisfactory efficiency gain, and need to use more expensive components. In cases where the feed of the inverter is interrupted by an electro-mechanical relay contact, there will be consumption by the bobbin of the relay in the intervals in which the compressor is connected. Thus, considering the conventional consumption of a relay at 260 mW, the gain for a cooling system with operating behavior similar to that of FIG. 3, will be 0.124 Wh, or 0.52%. More significant than this difference is the cost for the solution with relay, because besides the thermostat itself, it will have to have a voltage supply with this additional capacity of 260 mW. On the other hand, in a case where the feed of the inverter is removed by a TRIAC semiconductor, there will be losses by conduction in this component in the intervals in which the compressor is operating. Considering that the RMS input current of the inverter is 0.3 A (40 W, 230 V, power factor of 0.58), the loss by conduction of a TRIAC will be about 360 mW, that is, the gain for a cooling system with an operating behavior similar to that of FIG. 3 will be less than 0.10 Wh, or less than 0.4%. And just as in the case of the relay, there will also be the cost of a TRIAC, of its command circuit, and of a potential form of heat dissipation, since the conduction losses may overheat the TRIAC when the compressor demands greater powers and currents from the power grid.